How will this debt be paid off??

Inflation! 

By debauching the U.S. Dollar and upcoming tax increases. 

The bond buying opportunity of a lifetime that began in 1981 with 10 year Treasury Bonds yielding 15.84% and then its roller coaster decline ending in 2020 at 0.52%.  The U.S. is now entering a phase just the opposite with interest rate increasing in a saw tooth manner.  During growth phases in the economy expect higher rates especially for long term bonds and during economic declines expect higher lows.